Heavy Metal Removal from Wastewater by Adsorption with Hydrochar Derived from Biomass: Current Applications and Research Trends

Hydrothermal carbonization (HTC) is an innovative technique by which a wet feedstock is converting into valuable product, hydrochar, without drying as a pretreatment step. Over all other thermal converting process, HTC is counted as the most cost-effective and environmentally friendly technique. The purpose of this review is to highlight the current research trends of HTC and the application of hydrochar derived from biomass in heavy metal removal. This review provides an overview about HTC fundamentals including its ecotoxicology and the factors such as biomass type, reaction temperature, retention time, catalysis, and pH medium, which affect hydrochar composition and performance. Moreover, various modification agents in order to function the hydrochar was discussed. It is proposed that the co-hydrothermal carbonization of two or more biomasses with the addition of other materials can enhance the functional groups and change the features of the produced hydrochar. This paper also reviews the kinetic analysis and adsorption isotherm as well as the hydrochar applications in heavy metal removal. Acacia arabica waste is produced annually in huge amount, which adversely affect the environment. Up to now, it has no specific reuse in a proper way. In this respect, biomass such as Acacia arabica wastes can successfully be valorized by converting into hydrochar via hydrothermal carbonization. Moreover, application as a green adsorbent to remove heavy metal contamination from wastewater due to its high porosity and great surface area is counted as a promising technique that contributes to this field of research.